X34 ASIA
as latitude 30° or 31°, though the great triangle of the Syrian
Desert or the Hamad, which the ancient geographers included
in Arabia, lies to the north of this line.
The broad physical features of the peninsula are already
familiar—the plateau with its high western edge overlooking
the Red Sea and its long, gentle slope to the Persian Gulf. Actu-
ally the edge of the plateau is higher than its function merely
as an edge of an elevated block would warrant. Peaks in Midian
and Yemen rise to over 8,000 and even 10,000 feet and the
western and southern edges of the plateau appear as mountain
ranges, not only from the sea, but also from the interior. This
is due in part to the presence of volcanic piles, or of lava-flows
resistant to denudation which have protected the underlying
sandstones and in part to the different nature of denudation at
high levels producing jagged outlines which contrast with the
sand-strewn plateau slopes. The recent explorations of H. St.
J. B. Philby have shown that the general eastern slope is not as
simple as was supposed; a great ridge of highland 4,000-5,000
feet high, and with granite peaks rising to over 6,000 feet, crosses
the heart of the peninsula from northern Yemen to Sedeir.
Most of the sand of the plateau is derived from the disintegra-
tion—largely due to the alternate heating by the sun's rays
and the cooling at night of the rocks—of mcsozoic sandstones
which were deposited over the ancient crystalline massif.1
There are no really perennial rivers in Arabia ; their place
is taken by countless river-valleys (wadis), which carry water
after rainstorms. The high western edge of the plateau is
naturally the main water-parting. The wadis which descend
to the Red Sea have deeply eroded beds which form a great
obstacle to communication from north to south; at the same
time the waters are useless for navigation or irrigation. The
wadis which descend towards the Persian Gulf (though often
never reaching it), on the other hand, are long and shallow, their
floors are often so slightly depressed below the general level that
the traveller may cross them without being aware of their exist-
ence. Whereas the western wadis to the Red Sea are obstacles
to communication, the wadis to the east are of distinct value.
Though they may not carry permanent surface water, water
is at all times present below their beds and rnay be reached by
wells, and where the water level rises sufficiently near the surface
strings of oases are found. These broad, shallow valleys are
*' South-eastern Arabia or Oman is unique in that its tectonic features
are allied to those of Persia and Baluchistan to the north, See G, 3VL
Xees, 'The Physical Geography of South-eastern Arabia/ Geographical
Journal, Vol. LXXI, 1928, pp, 441-470.